[clang] [AMDGPU] Do not emit arch dependent macros with unspecified cpu (PR #80035)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> i.e. it helped with Clang :: Preprocessor/predefined-arch-macros.c but not with:
> 
> Failed Tests (2): Clang :: Driver/amdgpu-macros.cl Clang :: Driver/target-id-macros.cl

Pushed a fix, `check-clang` passes on my machine now. Let me know if it's still broken.

https://github.com/llvm/llvm-project/pull/80035